1. Field of the Invention
Aspects of the present invention relate to a pouch type lithium secondary battery, and more particularly, to a pouch type lithium secondary battery that can minimize a size of a cell.
2. Description of the Related Art
Generally, a lithium secondary battery uses a non-aqueous electrolyte because the lithium actively reacts with water. The electrolyte may be a solid polymer containing lithium salts or a liquid in which the lithium salts are dissociated from an organic solvent.
The lithium secondary battery can be divided according to a kind of electrolyte used, i.e., lithium metal and lithium ion batteries, both of which use a liquid electrolyte, and lithium ion polymer batteries, which use a solid polymer electrolyte. The lithium ion polymer battery can be divided into a complete solid type lithium ion polymer battery containing no organic electrolyte and a lithium ion polymer battery using a gel type polymer electrolyte containing an organic electrolyte.
In the lithium ion polymer battery, a multi-layer film pouch can be used instead of a metal can as used in the lithium metal and lithium ion batteries. The multi-layer film pouch may be formed of a metal foil and at least one polymer film covering the foil. When such multi-layer film pouch is used, the weight of the secondary cell can be greatly reduced compared to the use of a metal can. Aluminum is usually used as the metal forming the foil in the multi-layer film pouch. The polymer film that forms an inner layer of the pouch film protects the metal foil from the electrolyte and prevents an electrical short between electrode tabs.
The pouch type lithium secondary battery may be formed by sealing an electrode assembly, which includes a cathode, a separator, and an anode stacked and/or wound, in a pouch. Then, the sealed pouch type bare cell is manufactured by bonding upper and lower pouch films to open peripheral parts by heating.
In the pouch type bare cell, generally two electrode tabs connected to the electrode assembly extend from the electrode assembly to the outside of the pouch type bare cell. A protection tape is provided to a portion where the two electrode tabs and pouch contact, thereby keeping the two electrode tabs insulated from the pouch during sealing. A stepped part is formed for each electrode tab. The stepped part has a width narrower than that of the protection tape at a sealing portion where the protection tape is provided in the pouch. In the stepped part, a region corresponding to the electrode tab is less pressurized. Thus, a possibility of the electrical short between the metal layer of the pouch and the electrode tabs becomes low and sealing pressure is substantially applied to the peripheral region of the stepped part, thereby improving the safety of the sealing.
As the size of the pouch type lithium secondary battery is greatly minimized, the width of the battery in a longitudinal direction of a withdrawn surface of the electrode tab becomes gradually shorter. In the battery having the short width, a gap between the withdrawn electrode tabs becomes very narrow.
In the battery having a narrow gap between the electrode tabs, it is difficult to exactly set the electrode tab at a position where the stepped part is formed during a sealing process. Accordingly, when the electrode tab is not properly set at the stepped part-forming position, the electrode tab is electrically shorted or defects in sealing occur.
Particularly, in the battery having a short width between the electrode tabs, it is difficult to weld the electrode tabs to an electrode plate and maintain a proper seal around the electrode tab while reducing distribution of gaps between the electrode tabs.